The three-way catalyst has been widely used as the catalyst for purifying HC, CO and NOx in an exhaust gas from a motor vehicle. The three-way catalyst is composed of a porous oxide support such as alumina, ceria, zirconia, ceria-zirconia, etc., which supports platinum group metals, such as Pt, Rh, etc. The three-way catalyst oxidizes HC and CO and reduces NOx to purify these components. These reactions proceed most efficiently in such an atmosphere that oxidation components and reduction components exist in an approximately equivalent amount, and consequently, in the motor vehicles on which the three-way catalysts are mounted, the air-fuel ratio has been controlled such that combustion is carried out at about the theoretical air-fuel ratio (stoichiometric air-fuel ratio) (A/F=about 14.6±0.2).
The three-way catalyst, however, exhibits the problem that when an exhaust gas atmosphere shifts to a reducing side, sulfur oxide in the exhaust gas is reduced and emitted as H2S. Accordingly, it can be considered to additionally use oxide of Ni or Cu as a component of the three-way catalyst. Oxide of Ni or Cu forms SO3 or SO4 from SO2 in an oxidizing atmosphere, and stores sulfur components as sulfides such as Ni2S3, ex. in a reducing atmosphere, thereby restraining emission of H2S.
However, Ni or Cu is an environmental load substance putting so that, recently, there is the tendency of limiting the use thereof as the component of the catalyst for purifying an exhaust gas of a motor vehicle.
Publications of examined Japanese patent applications Nos. Hei02-020561, Hei05-081521, Sho61-020342, etc. disclose catalysts, each containing bismuth components and being capable of oxidizing H2S to eliminate the same. These catalysts, however, oxidize H2S in an oxidizing atmosphere so that, in the stoichiometric or reducing atmosphere, H2S may be emitted. These publications do not disclose the effects in the stoichiometric or reducing atmosphere, and neither disclose nor suggest the application of these catalysts to the three-way catalysts which are used in the stoichiometric or reducing atmosphere, too.
In addition, publications of unexamined Japanese patent applications Nos. Hei09-075739, Hei11-005035, etc. disclose catalysts in which porous oxides containing zinc oxide, tin oxide, etc. in addition to alumina, zirconia, etc. support platinum group metals. However, these catalysts have been used in exhaust gases containing not less than the theoretical reaction amount of oxygen so that they cannot be considered to be applied to the three-way catalysts.
[Patent Document 1] Publications of examined Japanese patent applications Nos. Hei02-020561
[Patent Document 2] Publications of examined Japanese patent applications Nos. Hei05-081521
[Patent Document 3] Publications of examined Japanese patent applications Nos. Sho61-020342
[Patent Document 4] Publications of unexamined Japanese patent applications Nos. Hei09-075739
[Patent Document 5] Publications of unexamined Japanese patent applications Nos. Hei11-005035